The functional and comparative morphology of the photoreceptors of the copepodid larva of the salmon louse Lepeophtheirus salmonis (Kroyer, 1837) (Crustacea : Copepoda, Caligidae)

The ultrastructure of the optic and extra-optic photoreceptors of the copep odid larva of the caligid copepod Lepeophtheirus salmonis (Kroyer, 1837) is described. The optic photoreceptor comprises a median nauplius eye consist ing of two lensed dorsolateral ocelli and a single unlensed ventral ocellus . The dorsolateral ocelli each comprise 9 retinula cells whilst the ventral ocellus comprises 10 retinula cells. All ocelli are bounded by a capsule o f conjunctival cells and the entire nauplius eye is surrounded by pigment c ells except for two fixed apertures lying above the anterodorsal lenses. An extraoptic photoreceptor is described for the first time in caligids; each bilateral organ comprises two cells resembling retinula cells and is locat ed in the anterior cephalothorax. The functional significance of the photor eceptors is discussed and theoretical abilities compared with known behavio ural responses.